TITLE: GCN CIRCULAR NUMBER: 41416 SUBJECT: GRB 250818A: Swift/UVOT Upper Limits DATE: 25/08/18 17:50:17 GMT FROM: Mike Siegel at PSU/Swift MOC mhs18@psu.edu

M. H. Siegel (PSU) and S. B. Cenko (GSFC) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250818A 162 s after the BAT trigger (Cenko et al., GCN Circ. 41403). No optical afterglow consistent with the XRT position (Osborn et al., GCN Circ. 41403) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are:

Filter T_start(s) T_stop(s) Exp(s) Mag

u (fc) 162 412 246 >20.3 b 418 438 19 >18.6 v 32212 32641 429. >20.2

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 1.602 in the direction of the burst (Schlegel et al. 1998).

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